The disclosure relates to a micromechanical sensor. The disclosure also relates to a method for producing a micromechanical sensor.
Micromechanical inertial sensors for measuring acceleration and rate of rotation are mass produced for various applications in the automobile and consumer sectors. For capacitive acceleration sensors with a direction of detection perpendicular to the wafer level (z direction), rockers are preferably used. The sensor principle of these rockers is based on a spring-mass system, in which in the simplest case a movable seismic mass with two counterelectrodes fixed on the substrate forms two plate capacitors. The seismic mass is connected to the substrate by way of a torsion spring. If the mass structures on the two sides of the torsion spring are of different sizes, under the effect of a z acceleration the mass structure will turn in relation to the torsion spring as an axis of rotation. Such a difference in mass is brought about for example by an additional mass, as shown in FIGS. 1 and 2. In this way, the distance between the electrodes becomes smaller on the side with the greater mass and greater on the other side. The change in capacitance is a measure of the acceleration acting. Such acceleration sensors are known from numerous patent specifications, for example from EP 0 244 581 and EP 0 773 443 B1.
The standard rockers are of a simple construction and are widely used, but have several technical problems that make them unsuitable for applications with very high requirements for offset stability, or that require very high costs, for example during packaging, in the application and when testing in order to make them suitable for high-performance applications. Among the effects that may adversely influence the offset or zero-point stability are the following:
a) The construction is sensitive to instances of substrate bending, for example caused by housing stress, in particular asymmetrical connections, in which for example an average distance between two electrodes varies. A resultant capacitance between the seismic mass and a first electrode then already deviates from the capacitance between the seismic mass and the second electrode without any acceleration signal, resulting in an offset signal. This offset is generally also dependent on the temperature, since housing stress and instances of substrate bending are temperature-dependent.
b) In the region of the additional mass, an undesired electrostatic force acts between the movable structure and the substrate, since an electrical voltage, for example a pulsed square-wave voltage, is applied to the movable structure for the capacitive evaluation, while the substrate is at ground potential. This force leads to a parasitic deflection of the rocker. To minimize the electrostatic interactions, usually arranged on the substrate in the region of the additional mass is an additional conductor track area, to which the same potential is applied as to the movable structure. Theoretically, this allows a freedom from forces to be achieved between the additional mass and the substrate. In practice, however, there may be significant electrical surface charges on the conductor track area connected to the substrate and/or on the underside of the movable structure, which can still lead to parasitic forces, and consequently offset signals. These effects are particularly critical if they change with temperature or over the lifetime of the product, since this leads to offset drifts, which cannot be corrected by the final adjustment of the component.
Surface-micromechanically produced z acceleration sensors according to the rocker principle generally have perforation holes both in the region of the electrodes and in the region of the additional mass. The holes are required in the case of most methods of surface-micromechanical production because these holes represent access channels for the so-called gas-phase etching, in which the sacrificial oxide between the movable structure and the conductor track level lying thereunder or the substrate is removed by way of a gaseous HF atmosphere to release the sensor.
The perforation holes also have the advantage in many cases that, with a multi-channel sensor (xz, yz or xyz sensor), the damping of the z sensor does not become too great, and in particular will not be significantly above the damping of the lateral sensor. It is advantageous if, with a multi-channel sensor, the mechanical transmission functions (i.e. resonant frequency and damping) of all the channels lie in a similar range. As a result, specific requirements for the mechanical bandwidth and/or vibrational robustness can be satisfied particularly well and without differentiation between the individual channels in the specification.
On the other hand, an advantage of a non-perforated mass structure is the greater mass per unit area, which leads to an increased mechanical sensitivity (or else with the same sensitivity to a reduced space requirement). The higher mass density of a non-perforated structure can alternatively be used beneficially for increasing the spring stiffness, and consequently for reducing the stiction tendency of the sensor.
To improve the mentioned disruptive effects a), b), some years ago novel z sensor designs and technologies were proposed, disclosed for example in DE 10 2009 000 167 A1. A structure disclosed therein displays a significantly improved robustness with respect to instances of substrate bending (differential electrode principle comprising a top electrode and a bottom electrode) and with respect to surface charges on account of the symmetrization of the underside of the movable structure with respect to the conductor track level).
To provide an overview, the strengths and weaknesses of the various z sensor concepts according to the prior art are summarized in the following table:
TABLE 1Rocker accordingNon-perforatedStandard rockerto FIG. 4rockerRobustness with−+−respect to substratebendingRobustness with−+−respect to surfacechargesDamping++−adjustable andcomparable to x/ysensorMechanical0−+sensitivityRestoring forceSmall overall00/+ (+ top/bottom0/+heightelectrodes, butadditional masseson both sides ofthe rocker+ . . . satisfied− . . . not satisfied0 . . . partially satisfied